440 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Histologically essentially the same conditions are found as in the preceding 

 stage, but all of the cells have become smaller as a result of the continued division. 

 In the ectoderm the traveling of the nuclei toward the inner ends of the cells has 

 made progress. On the outer side of the ectoderm, between the long narrow pris- 

 matic cells, lie the globular cells in process of division. 



The entoderm cells are of very different shapes and lengths; some of them, 

 lying on the lateral borders, exceed considerably the longest ectoderm cells. Lat- 

 erally and posteriorly the primitive gut is composed of a single layer of cells, and is 

 sharply differentiated from the body cavity. While in this region only isolated 

 cells sporadically leave the entodermal epithelium to become mesenchvme cells, this 

 process continues very actively and uninterruptedly over the anterior end of 

 the gut. 



The mesoderm cells, which now fill the major portion of the body cavity, are 

 mostly more or less spherical in shape, with the nuclei either resting or undergoing 

 division. 



In a slightly more advanced stage mesoderm cells have pushed themselves in 

 between the posterior portion of the primitive gut and the ectodermal layer, and 

 the furrow which indicated the course of the blastopore has disappeared save for a 

 slight indication in two or three sections. 



In other embryos of this same age the course of the original blastopore can no 

 longer be made out on the spherical body wall, though its location may readily be 

 determined through the close approximation of the now detached primitive irut to 

 the ectodermal wall. 



Often at this stage the two crossed axes can not with certainty be determined, 

 in which case the embryo again exhibits an almost monaxial structure which is dis- 

 turbed only by the slight displacement of the gut sack ventrally and toward the left. 

 In numerous embryos, however, the detached primitive gut remains in the trans- 

 verse axis and the blastopore slit is somewhat more elongated and persistent, so that 

 bilaterality is demonstrable. Both types are found side by side on the same mother, 

 and it may be that the first condition is a more advanced stage of the second. 



DIVISION OP THE PRIMITIVE GUT INTO MESENTEHON, PERITONEAL SACK, AND PRIMARY HYDROCrELE. 



During the second night the division of the primitive gut into two sections 

 begins. This is effected through the appearance of a groove encircling the gut sack 

 at right angles to the major axis of the embryo, which presses the cells inward before 

 it, forming a sphincter-like constriction. As the cells are pressed inward the com- 

 munication between the two sections of the gut becomes more and more restricted 

 until finally it is entirely interrupted. The last trace of this intercommunication 

 always lies in, or very near, the chief axis. 



The two sections of the primitive gut always differ in size, though this differ- 

 ence is subject to individual variation and is often very slight. The anterior section 

 is the larger, later becoming the mesenteron and hydrocrele, while the smaller hinder 

 section gives rise to the ccelome. 



Only at the very first appearance of the groove do the lumina of the two por- 

 tions of the gut appear similar, in sections cut across the chief axis as a series of 



